On Thursday, the California Geological Survey released new tsunami hazard maps for San Francisco, Contra Costa and Santa Clara counties, along with several in Southern California, for the first time since 2009. Updated with new technology and knowledge gained from major events like Japan's catastrophic Tohoku earthquake and tsunami in 2011, the interactive maps show flooding risk in a wider part of San Francisco's downtown, Marina and North Beach neighborhoods and areas around the Port of Richmond than previously anticipated.
The danger areas reflect a worst-case scenario: a tsunami resulting from a magnitude 9.3 earthquake in the eastern Aleutian Islands off of Alaska, the location that presents the greatest risk of such destructive waves hitting Central California. Such an event would be extremely rare — and would take five hours to reach the Bay Area, so there would be time to warn people to get to safety, said Rick Wilson, senior engineering geologist at the California Geological Survey, part of the state Department of Conservation.
"So 99% of the time we're not going to see this tsunami," said Wilson, a member of the team updating the maps. "But it's good to prepare for the worst and hope for the best."
In March, the state released updated tsunami hazard maps for San Mateo and Alameda counties, and will do the same for the rest of the Bay Area and state by early next year.
If an earthquake occurs that could cause a tsunami, the National Tsunami Warning Center provides alerts, as do local governments, with everything from bullhorns to Amber Alert-style text messages indicating if an evacuation is warranted. There are different tiers of risk — while a tsunami advisory warns people to stay off beaches and harbors, a tsunami warning prompts inland evacuation.
The maps from the California Geological Survey are meant to warn people in advance if they're in a tsunami hazard zone and how far they'd need to go reach safety.
"You don't have to run to the mountains to be safe," Wilson said.
This time, Wilson and his colleagues used lidar mapping technology, which has higher resolution than what was previously used, and it pinpointed more low-lying areas susceptible to flooding.
They include parts of downtown San Francisco, where the hazard zone now extends inland as far as Fremont Street in the South of Market area and Sansome Street in the Financial District. In North Beach, the zone has been extended from Beach Street to as far south as Chestnut at Columbus. At Ocean Beach, the San Francisco Zoo is a newly named tsunami evacuation zone, prompting visions of Noah's Ark.
Wilson said waves in a worst-case scenario tsunami could be as high as 30 feet at the coast and 10 to 15 feet inside the bay. If such a tsunami overcame San Francisco's waterfront, for example, water could be overhead at the edge of the bay and dissipate to about 2 feet at the outer reaches of the hazard zone, still enough to knock people down, he said.
However, any work being done to protect the coast from sea level rise could also help protect it from a tsunami, Wilson said. In San Francisco, plans under way for a $3 billion upgrade to the Embarcadero sea wall will help prepare for both sea level rise and tsunami risk, said Adrienne Bechelli, deputy director of the San Francisco Department of Emergency Management.
"This is a great example of where a capital investment has a multitude of benefits for all the different hazards and risks," she said.
Bechelli said city departments, from transit to Public Works, hold drills to prepare for events like a major tsunami. In addition, her department is working with the Recreation and Park Department on a worst-case evacuation plan for the zoo.
In Contra Costa County, previous maps of the Port of Richmond showed the tsunami hazard to be mostly at the port itself and the harbor next to it, while the new maps extend the evacuation zone well north of Highway 580. In Santa Clara County, flooding risk is low because a major tsunami would dissipate to about 2 feet by the time it reached the South Bay, and existing levees and ponds would protect residential and business areas, Wilson said.
Updated maps of Alameda County, which were released in March, show wider hazard zones in West Berkeley, extending as far east as Sixth Street. In Oakland, new areas include Lake Merritt and a small region east of the Bay Bridge.
Tsunamis can hit the California coast from several sources, both near and far. An earthquake off of Point Reyes could cause a small local tsunami, and Japan's 2011 tsunami killed one person in California and caused $100 million in damage to the state's ports and harbors. But a major earthquake in the Aleutian Islands poses the most risk because the fault there is in a subduction zone — the type most likely to cause a tsunami — and resulting waves would head straight to Central California.
In 1964, a 9.2 magnitude earthquake in a nearby part of Alaska caused a tsunami with surges 21 feet high that killed 12 people and caused major destruction in Crescent City ( Del Norte County). Japan's 2011 earthquake and tsunami, which killed close to 20,000 people, was a 9.0 quake on a subduction zone only 80 miles east of Japan. The tsunami took only 20 to 30 minutes to reach land, whereas California would have a much longer warning.
"We do want people to see this is a hazard," Wilson said. "But we want them to have hope that there are things they can do to take care of business."
Tara Duggan is a San Francisco Chronicle staff writer. Email: tduggan@sfchronicle.com Twitter: @taraduggan
National Tsunami Warning Center provides alerts: tsunami.gov
California Tsunami Preparedness Guide. Tsunami hazard maps: www.tsunami.ca.go
For emergency alerts text your ZIP code to 888-777
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